1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to circuit interrupters including a limiter. The disclosed concept also pertains to limiters.
2. Background Information
Current limiters employ a current interruption device that rapidly and effectively brings the current to a relatively low or zero value upon the occurrence of a fault or another overload condition.
Circuit protection devices protect electrical equipment from damage when excessive current flows in a power circuit. Such devices have a relatively low resistivity and, accordingly, a relatively high conductivity under normal current conditions of the power circuit, but are “tripped” or converted to a relatively high or complete resistivity when excessive current and/or temperature occurs. When the device is tripped, a reduced or zero current is allowed to pass in the power circuit, thereby protecting power circuit conductors and corresponding load(s) from electrical and thermal damage.
Conventional circuit interrupters, circuit protection or current limiting devices include, but are not limited to, circuit breakers, fuses (e.g., expulsion fuses), thermistors (e.g., PTC (Positive Temperature Coefficient) conductive polymer thermistors), and the like. These devices are current rated for the maximum current the device can carry without interruption under a load.
For example, circuit breakers typically contain a load sensing element (e.g., a bimetal; a hot-wire; a magnetic element) and separable contacts, which open under overload or short circuit conditions. Most circuit breakers have to be manually reset either locally at the circuit breaker or through a remote switch.
Fuses typically contain a load sensing fusible element (e.g., metal wire), which when exposed to current (I) of fault magnitude rapidly melts and vaporizes through resistive (R) heating (I2R). Formation of an arc in the fuse, in series with the load, can introduce arc resistance into the power circuit to reduce the peak let-through current to a value significantly lower than the fault current. Expulsion fuses may further contain gas-evolving or arc-quenching materials which rapidly quench the arc upon fusing to eliminate current conduction. Fuses generally are not reusable and must be replaced after overload or short circuit conditions because they are damaged inherently when the power circuit opens.
Low voltage circuit breakers are often connected in series with current limiters, in order to significantly increase the short circuit switching capacity in low voltage electrical networks and to significantly limit cut-off currents. See, for example, U.S. Pat. Nos. 7,558,040; and 7,362,207. Such limiters are designed to transition rapidly, in case of a short circuit, from a low-resistance state to a high-resistance state and, thus, provide rapid current limiting and disconnection. Some limiters employ, for example, fuses, such as fusible wire elements to accomplish this function.
Many known limiters are fused devices (non-reusable) that do not have out-gassing concerns.
If a limiter could be coupled to, for example, the line end of a circuit breaker, then it would essentially block an ionized gas stream that comes from the circuit breaker during a fault interruption.
A number of circuit breaker manufacturers vent ionized gas from the line end of their circuit breakers. Other manufacturers of circuit breakers employ a double-break contact system with multiple arc chutes that can have ionized gas venting out both ends of the circuit breaker during a fault interruption.
There is room for improvement in limiters for electrical switching apparatus.